Stray start tag “h2”.

An <h2> that comes after a </body> or </html> end tag is stray, because the HTML parser accepts no more elements once the body has ended.

Once the parser reads </body>, only whitespace, comments and the closing </html> tag may follow. Any element after that point is a parse error. Browsers recover by treating the heading as part of the body again, so the page usually looks right and the mistake goes unnoticed. After </html> the validator also gives up: it reports "Cannot recover after last error" and checks nothing further down the page, so other errors stay hidden until you fix this one.

The </body> that ends the body is often not the one at the bottom of the page. A block of HTML included from a standalone file, such as a widget or a newsletter form, can bring its own <html>, <body>, </body> and </html> tags. The parser ignores the extra start tags, which the validator reports a few lines earlier, but it obeys the end tags, so the real body ends in the middle of the page. The <h2> that opens the next section is then the first element after that point. A template that closes the body before printing the rest of the content has the same effect.

Remove the <html>, <head> and <body> tags from the included block so only its content reaches the page, and keep one </body> and one </html> at the very end of the document. If the heading sits after the page's own </body>, move it inside the body.

Invalid example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>News</title>
</head>
<body>
  <main>
    <h1>News</h1>
    <!-- Included newsletter block -->
    <html>
    <body>
      <p>Subscribe to our newsletter.</p>
    </body>
    </html>
    <h2>Latest articles</h2>
    <p>New articles appear here every week.</p>
  </main>
</body>
</html>

Valid example

<!DOCTYPE html>
<html lang="en">
<head>
  <title>News</title>
</head>
<body>
  <main>
    <h1>News</h1>
    <!-- Included newsletter block -->
    <p>Subscribe to our newsletter.</p>
    <h2>Latest articles</h2>
    <p>New articles appear here every week.</p>
  </main>
</body>
</html>

Learn more:

MDN Reference

MDN Web Docs: Heading elements

→

MDN Reference

MDN Web Docs: <body>

→

WHATWG Spec

HTML Standard: Parsing HTML documents

→